1. Field of the Invention
The present invention relates to an image forming apparatus for printing on a the front and back of paper, and more particularly to an image forming apparatus for printing on the front and back of paper having a size exceeding the maximum paper-feed size.
2. Description of the Background Art
Electrophographic image forming apparatuses include, for example, MFP (Multifunction Peripheral) with a scanner function, a facsimile function, a copy function, a printer function, a data communication function, and a server function, facsimile machines, copiers, and printers.
Image forming apparatuses have the maximum paper size that can be fed (for example, A3 size or A4 size). This size is called the maximum paper-feed size. In order to print on paper having a size exceeding the maximum paper-feed size, a more expensive and larger image forming apparatus (such as a copier or a printer) that supports such size of paper has to be used. In this respect, a technique for printing on paper having a size exceeding the maximum paper-feed size has been proposed, in which paper folded in two is fed. For example, Document 1 below discloses a technique of forming an image on large-size recording paper by folding recording paper in two, forming toner images on both sides of the recording paper folded in two, and unfolding the recording paper after fixing.
Documents 2 and 3 below disclose a technique related to operation of a paper feeder in feeding paper folded in two. Document 2 below discloses a paper feeder including a paper stage on which paper is placed, a separation paper feed roller pair located downstream of the paper stage for feeding paper, and a bringing roller coming into abutment with the upper surface of paper placed on the paper stage for bringing the paper closer to the separation paper feed roller pair. This paper feeder can switch between a first mode in which the rollers of the separation paper feed roller pair are rotated in a direction in which paper is output to the downstream side and the bringing roller is not brought into abutment with the upper surface of paper, and a second mode in which rotation of a separation roller of the separation paper feed roller pair is stopped and the bringing roller is brought into abutment with the upper surface of paper.
Document 3 discloses a sheet feeder in which when the conveyed sheets overlie one another, separation means separates the sheets one by one. This sheet feeder allows the separation means to stop the separating operation when information about the conveyed sheet indicates a folded sheet.    Document 1: Japanese Laid-Open Patent Publication No. 11-327364    Document 2: Japanese Laid-Open Patent Publication No. 2003-12169    Document 3: Japanese Laid-Open Patent Publication No. 2007-302376
Conventionally, when double-sided printing is done on paper having a size exceeding the maximum paper-feed size, an image forming apparatus does printing in the following manner. The image forming apparatus does printing, first, on the right half of the front and then on the left half of the front of paper folded in two with the front facing outside. The image forming apparatus then does printing on the right half of the back and finally on the left half of the back of the paper folded in two with the back facing the outside.
In the conventional techniques, when double-sided printing is done on paper having a size exceeding the maximum paper-feed size, the efficiency in printing is poor because the image forming apparatus has to make, in total, at least four prints as described above.